The invention relates generally to marine propulsion devices such as stern drive units and outboard motors, and more particularly to such marine propulsion devices with trim tabs or exhaust snouts.
Trim tabs have been employed for some time in stern drive units to counter "steering torque", i.e., the tendency of a drive shaft housing to rotate due to the torque applied to the drive shaft, and thus, to cause an undesired steering effect. Trim tabs have concurrently served as exhaust gas discharge snouts and have also been employed with so-called "through-the-propeller-hub" exhaust gas discharge systems.
Examples of prior trim tab construction are disclosed in the U.S. Shimanckas Pat. No. 3,000,183, issued May 19, 1965, and in the U.S. Pat. No. 3,537,419, to Holtermann issued Nov. 3, 1970.
Another prior trim tab construction is disclosed in the Canadian Pat. No. 687,868, to Kiekhaefer issued June 2, 1964.
Still another prior trim tab construction is disclosed in the U.S. Pat. application Ser. No. 26,234, to Granholm filed June 26, 1972.
The amount of counteraction to the "steering torque" developed by a trim tab can vary depending upon the pressure distribution produced by the water flowing past the trim tab. The pressure acting on a portion of the trim tab may be below atmospheric pressure. If air leaks or migrates into this low pressure area and "ventilates" it, the pressure is raised and the effectiveness of the trim tab in counteracting steering torque is reduced.